viir.l WOOD. 119 



trees than in the Oak, the wood of the common European 

 species of which, when cut lengthwise in the direction of 

 these rays, is marked by silvery patches of the cells of the 

 rays, forming what is called the silver-grain, which painters 

 imitate in painting wainscot. 



It will be difficult to find the cambium-ring without using 

 a lens, but it is immediately within the bark, which it con- 

 nects with, while at the same time it separates it from, the 

 wood. 



The outer layers of bark are usually composed of short 

 cells of corky texture, which serve to prevent the cambium- 

 layer from drymg up, by checking evaporation from the 

 surface. 



The inner or liber-layer of the bark of many plants and 

 trees is made use of for cordage and in cloth-making. 

 Hemp, sunn-hemp, jute, and flax, are all derived from this 

 layer, which, in the plants affording these products, is 

 very tough. 



15. The questions now present — Through which of these 

 cells, or systems of cells, in the stem, is the watery sap 

 absorbed by the roots conveyed to the leaves ? And through 

 which are the products, elaborated in the leaves under the 

 influence of light, conveyed to their respective destinations 

 for present or future use ? In other words, How do the 

 sap and nutrient fluids circulate in the plant? But these 

 questions, reasonable though they seem, it is im. possible to 

 answer fully in the present state of our knowledge. 



It is not our business just now to concern ourselves with 

 contested points ; so we must be content with a very general 

 and partial explanation. In the first place, we must recall 

 the fact that the entire plant is built up of closed cells and 

 vessels ; consequently, solid substances, even in the minutest 

 state of subdivision and suspension in water, cannot be 



